SU carburetors - reservoirs for oil? Why?

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I was talking with the owner of a '73 Datsun 240Z, and he mentioned that he'd replaced the original SU carburetors, and that they (the SU carbs) had had reservoirs for oil that had to be topped up regularly.

An internet search turns up lots of discussions about what kind of oil, etc., but nowhere did I find an explanation of why oil is required in a carburetor.

Can anyone explain this to me? Thanks.
 
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From memory a few decades ago. The SU carburetors had a tapered needle attached to the carburetor slides. The fuel came into the airflow in a fixed orfice at the bottom of the carburetor. The tapered needle moved in and out of the orfice to vary the mixture to the throttle opening and load. Oil was a damper to the slide holding the tapered needle.
 

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The oil is used to damp the slides. That way they don’t vibrate up-down with every intake stroke
From memory a few decades ago. The SU carburetors had a tapered needle attached to the carburetor slides. The fuel came into the airflow in a fixed orfice at the bottom of the carburetor. The tapered needle moved in and out of the orfice to vary the mixture to the throttle opening and load. Oil was a damper to the slide holding the tapered needle.
I had never heard of carburetor slides - it looks like they're an alternative to a conventional butterfly valve.
 
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The oil in the damper pot has an impact on the "accelerator pump effect"

CV carbs don't have an accelerator pump. Accelerator pumps generally provide a brief mixture enrichment during sudden throttle openings for increased acceleration. They are usually mechanically operated but the principle of the CV carb produces an effect very similar to that of a traditional pump. Because of inertia and the damper oil, the piston in a CV carb does not rise as quickly as air velocity increases when the venturi is suddenly exposed to greater vacuum (throttle opening). The temporarily increased air speed in the venturi enrichens the mixture. The result is increased acceleration without the mechanical complexity of an accelerator pump.

The viscosity of the damper oil will determine the degree of accelerator pump effect so you may wish to experiment.
 
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The SU side draft carb is simple as a clothes pin. They work well if the slide is smooth, the bore round and the needle not bent. I always used 10W in mine.

su carb.jpg
 
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The oil in the damper pot has an impact on the "accelerator pump effect"

CV carbs don't have an accelerator pump. Accelerator pumps generally provide a brief mixture enrichment during sudden throttle openings for increased acceleration. They are usually mechanically operated but the principle of the CV carb produces an effect very similar to that of a traditional pump. Because of inertia and the damper oil, the piston in a CV carb does not rise as quickly as air velocity increases when the venturi is suddenly exposed to greater vacuum (throttle opening). The temporarily increased air speed in the venturi enrichens the mixture. The result is increased acceleration without the mechanical complexity of an accelerator pump.

The viscosity of the damper oil will determine the degree of accelerator pump effect so you may wish to experiment.
I've found it interesting how different engines respond to different weights of oil in the dampers, and some of the US spec smogged cars (my Midget 1500 for one) seemed much more sensitive. I now use ATF in most of mine.
 
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We mainly used AFT...but if you had a SU on a different application, and you couldn't get a needle to suit you could try different oils to compensate...and the spring strength is another variable. I have a small booklet of all the SU needles, and all the measurements along their length...10.
 
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Like a lot of other types of oil, there are "thick" and "thin" oil arguments for SUs.

I'm personally in the thick camp. The whole purpose of the dashpot is richen the mixture on acceleration(or as folks who've really looked at AFRs during acceleration tell me, they will always go lean, but the dashpot makes them go "less lean"). Thicker oil slows the rise of the piston, which should increase the "accelerator pump" effect of the carb. I run 20W-50 in mine, and as much as anything because that's what I'm already putting in the engine. One reputable MG mechanic I know(John Twist) swears by 90wt gear oil, which at typical carb temperatures has a viscosity similar to 20W-50 or SAE30. I've gone as high as SAE60, but didn't really see any difference and that's not easy to find.

I constantly hear of the people who use ATF, MMO, or the like having to top them up. With engine oil, I don't think I've ever had to add any unless I intentionally dumped it out.

By the way, after putting a hotter cam in my MG, I had a terrible time tuning mine. If I got it to where it was somewhere in the ballpark of correct at speed, it would be so rich at idle that it would coke up the plugs in no time. I talked to Joe Curto, one of the SU carb masters, and he sent me AAA needles for my HS4 carbs. That's considered a "richer" needle than what I had, and it's a bit counter-intuitive to think that would fix the problem but it did. They're still not perfect but they are a LOT better.
 
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That's true. I always hated the need to constantly "top up" my SU.
But when I did, the performance was quite noticeable.
 
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I had never heard of carburetor slides - it looks like they're an alternative to a conventional butterfly valve.
On some carbs, mostly cycle yes, but on SU and ZS, no. You still have a butterfly which controls the vacuum seen by the top of the slide (and various other ports) and moving it.
 
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I had never heard of carburetor slides - it looks like they're an alternative to a conventional butterfly valve.

SUs and other CV carbs like ZSs have throttle plates.

It's not exactly like a motorcycle carb where the throttle moves the needle directly.

Instead, the piston reacts indirectly so as to vary the throat diameter and keep the velocity(and vacuum) through the throat constant. As the piston moves upward, the tapered needle is pulled further and further out of the jet, effectively increasing the jet diameter. I know that part is similar to a slide carburetor, but again the key is it is in response to engine demand and not directly controlled by the operator.

The whole design is honestly so elegantly simple. In addition, at least in theory it is able to meter fuel much more precisely than a typical fixed venturi carb. An SU needle has 8 "stations" to cover different demand situations.
 
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I had never heard of carburetor slides - it looks like they're an alternative to a conventional butterfly valve.
SU carbs were found only on British cars and under license by Hitachi for Nissan’s Z-cars in the 1970s.

Keihin and Mikuni took the SU slide design - a variable venturi carb and used a spring-loaded diaphragm vs the oil dampers. It was used from the 1970s and still used by the Japanese on off-road dirt bikes. I think Harley even used Keihin carbs.
 
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Well I learned something today! Great thread.

I'm very familiar with the CV (constant velocity) carb setup as I worked on Japanese motorcycles for a living many years ago. Many slight variations to the design, but the tapered needle in jet design is very intuitive. Some do have a mechanical accelerator pump, some have rubber diaphrams, some are just a tightly machined aluminum barrel, some slides are flat (attached directly to throttle cable), and some are round (with conventional butterfly throttle valve upstream).

The Japs copied someone else's design.... SU.
 
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Well I learned something today! Great thread.

I'm very familiar with the CV (constant velocity) carb setup as I worked on Japanese motorcycles for a living many years ago. Many slight variations to the design, but the tapered needle in jet design is very intuitive. Some do have a mechanical accelerator pump, some have rubber diaphrams, some are just a tightly machined aluminum barrel, some slides are flat (attached directly to throttle cable), and some are round (with conventional butterfly throttle valve upstream).

The Japs copied someone else's design.... SU.
IIRC, the Japanese, specifically Hitachi, licensed the rights from SU or whoever held the IP then for their automotive carbs. Ironically the Zenith Stromberg design was more of a 'copy', but utilized a diaphragm to seal the vacuum chamber and some other differences to operate outside of the SU patents.

I thought the slide as throttle design may have been developed by one of the Japanese makers, but Nthach is probably correct.
 
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I always admired the simple but clever method by which an Su carb produced cold start enrichment. Remember old motorcycle Amal carbs that had ticklers. They depressed the float to flood or increase the fuel level in the bowl. The higher fuel level enriched the mixture because it was easier for the venturi vacuum to draw fuel up through the needle jet.

Instead of increasing the fuel level, what SU did was to lower the needle jet which amounted to the same thing. So when you pulled the choke control on an old car with an SU carb, what you were actually doing is actuating a linkage to physically lowering the needle jet closer to the float bowl fuel level.
 
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